Air classifiers serve to separate (classify) the feed material dispersed in a fluid into a fine and a coarse fraction. The separation effect of a classifying wheel of the given type is based on the fact that the centripetal force of the fluid and the centrifugal force in the flow channels between the classifying wheel vanes of the so-called deflector wheel affect the individual particles of the product in opposite directions. With small particles, the centripetal force predominates so that the particles become entrained in the fluid and are discharged as fines. In the case of large particles, the centrifugal force predominates so that they are catapulted against the fluid flow out of the deflector wheel. The particle size where the centrifugal force and centripetal force are in equilibrium, i.e. where the particles have the same probability of entering the fine or the coarse fraction, is called the cut point size.
The demands regarding the classification of bulk materials are increasing constantly. Increasingly high amounts of bulk materials are meanwhile being classified. And the demands placed on the classification results are also becoming higher and higher. It is not enough these days for the classification to be economically efficient, a high cut point and yield are also demanded.
One of the problems encountered when classifying abrasive products is the wear to the classifying wheels and especially to the classifying wheel vanes.
In the case of extremely small classifying wheels, exchange of the complete classifying wheel is still acceptable because classifying wheels are used which are in one-piece design and which consist of one single sintered material such as ceramic, as disclosed in German patent DE 41 40 656 A1.
With larger classifying wheels, exchangeable classifying wheel vanes are employed. This makes for inexpensive servicing of the worn classifying wheels, because it is not the complete classifying wheel which needs exchanging but rather only the worn classifying wheel vanes. The classifying wheel vanes are preferably made of wear-resistant ceramic materials. This type of classifying wheel is disclosed in German patent DE 196 13 902 A.
In order to achieve the sharpest possible separation into a fine fraction and a coarse fraction, it is essential that a constant flow pattern at a uniform average radial speed of the fluid is present in all the flow channels between the classifying wheel vanes. If this is not possible, classifying wheels with extensions are employed, as are disclosed in German patent DE 198 40 344 A. The extensions are installed inside the flow channels between two classifying wheel vanes. The extensions prevent undesirable turbulence inside the flow channels, even if an undirected flow is present at the outer periphery of the classifying wheel. As a result of the complex design of these classifying wheels and the desired operating speeds, a wear-protected classifying wheel design has not yet been employed.